A transparent optical material and a plastic lens are molded articles required to have high transparency and optical is apparent from the uses thereof. As a material conventionally used widely in these uses, there is listed a plastic material obtained by radical polymerization of diethylene glycol bis(allyl carbonate) (hereinafter, abbreviated as DAC).
However, this DAC plastic material has low refractive index (Nd) as low as 1.50, and when used for a plastic lens, there is a problem that the resulting lens has large edge thickness and is deficient in fashionable appearance.
For improving this problem, various investigations have been made for increasing refractive index. For example, there are listed a method in which tetrabromo bisphenol-A is reacted with an isocyanate compound (Japanese Patent Application Laid-Open (JP-A) No. 58-164,615), a method in which xylylenedithiol dimethacryalte is polymerized (JP-A No. 64-31,759), a method in which 1,4-dithiane-2,5-dimercaptomethyl is used (Japanese Patent Application Publication (JP-B) No. 6-5,323), and the like.
Also the present inventors have previously suggested methods in which a polythiol having in the molecule a sulfur atom other than a mercapto group is used (JP-A Nos. 2-270,859 and 7-252,207), and the like.
Any of these methods is a method in which refractive index is improved by introducing a benzene ring, a halogen atom such as bromine, or a sulfur atom into the molecule. However, when refractive index is desired to be further improved, these methods have been often insufficient. That is, these methods have not been recognized as methods fully satisfying eager requirement to further decrease the edge thick of a lens.